Question: Stephanie is 30 years older than Tiffany. Thirteen years ago, Stephanie was 4 times as old as Tiffany. How old is Tiffany now?
Explanation: We can use the given information to write down two equations that describe the ages of Stephanie and Tiffany. Let Stephanie's current age be $s$ and Tiffany's current age be $t$ The information in the first sentence can be expressed in the following equation: $s = t + 30$ Thirteen years ago, Stephanie was $s - 13$ years old, and Tiffany was $t - 13$ years old. The information in the second sentence can be expressed in the following equation: $s - 13 = 4(t - 13)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $t$ , it might be easiest to use our first equation for $s$ and substitute it into our second equation. Our first equation is: $s = t + 30$ . Substituting this into our second equation, we get the equation: $(t + 30)$ $-$ $13 = 4(t - 13)$ which combines the information about $t$ from both of our original equations. Simplifying both sides of this equation, we get: $t + 17 = 4 t - 52$ Solving for $t$ , we get: $3 t = 69$ $t = 23$.